The invention relates to a heating apparatus for an enclosure having at least a ceiling and a side wall, apparatus comprising an electric heating element and support means therefor, and in particular although not exclusively, such a heating apparatus for a kiln for firing ceramics.
Many forms of heating apparatus for use in enclosures such as kilns and furnaces are known. All of these prior all forms of heating apparatus suffer from disadvantages of one kind or other.
In general heating enclosures such as kilns are provided with a plurality of heating apparatus. Each heating apparatus preferably forms a separate electrical circuit, such that if one fails the effect on the firing of the kiln is not too great, but several may be incorporated in one electric circuit. Hence, it is not desirable for the electrical elements used to have too large a cross sectional area as that limits the number which can be used.
One form of prior art heating apparatus comprises strip or ribbon heating elements. On the walls of enclosures these may simply be hung over support means of various kinds protruding from the ceiling and/or wall, and in general hang down between successive support means. On the ceiling of the enclosure they may be wound around two spaced apart lines of support means, protruding from the ceiling, in a zig-zag formation parallel to the ceiling, or draped over support means suspended from the ceiling, and in general hang down between successive support means in a similar fashion to the walls. On the floor of an enclosure they may simply be laid in a zig zag fashion in troughs provided in the floor. However, strip or ribbon heating elements have a relatively large cross sectional area and therefore only a relatively small number of circuits can be provided.
An alternative form of prior all heating apparatus provides rod elements, which may be supported in the same ways as the strip or ribbon elements previously described. These suffer from the same disadvantages as the strip or ribbon elements.
Many prior art heating apparatus comprise heating elements in the form of wire spirals or coils. These may be supported in channels provided in the walls, ceiling or floor of the enclosure, or on trays supported from the appropriate surface of the enclosure. Such heating elements suffer from the disadvantage that as they under go heating and cooling cycles they migrate towards cold areas of the enclosure, for example towards the enclosure door. This results in an uneven distribution of the coil loops along the length of the coils. This in turn produces a number of deleterious effects. First, some parts of the enclosure are heated to a higher temperature than others resulting in uneven firing. Second, where the loops of the coil are so tightly bunched as to approximate to a tube the heat transfer is poor, which produces generally inefficient heating thus requiring greater power input, and shortening heating element life as they may reach higher temperatures than they are designed to withstand. Third, where such elements are supported on trays these often suffer from an excess of heat and start to warp thus requiring replacement.
Wire spirals or coils may also be provided in free radiating form, that is, wound around supporting rods which are themselves supported from the appropriate surface of the enclosure. These elements suffer from the same kinds of disadvantages.